The present application is based on Japanese priority application No.11-304628 filed on Oct. 26, 1999, the entire contents of which are hereby incorporated by reference.
The present invention generally relates to semiconductor devices and more particularly to a ferroelectric capacitor, a semiconductor device having such a ferroelectric capacitor, and a fabrication process thereof.
Conventionally, EPROMs and flash memory devices are used extensively as a non-volatile semiconductor memory device that retains information even when electric power is turned off. These EPROMs and flash memory devices have a floating gate electrode and a tunneling insulation film cooperating therewith for retaining information. Particularly, a flash memory device has a simple construction in that only a single memory cell transistor is included in a single memory cell similarly to the case of a DRAM, and is thus suitable for constructing a large-scale integrated circuit.
In a flash memory device, in which writing or erasing of information is achieved by way of hot-electron injection or removal to and from the floating gate electrode via the tunneling insulation film, it is necessary to use a high voltage at the time of writing or erasing operation. Thus, it is inevitable that a large electric field is applied to the tunneling insulation film, while such a large electric field induces deterioration of the tunneling insulation film and limits the lifetime of the flash memory device. Further, in view of the fact that writing of information is achieved by way of injection of hot electrons, it takes a considerable time for writing information in flash memory devices. While a flash memory device is capable of storing multi-value information by controlling the amount of the electric charges injected into the floating gate electrode, such a possibility of multi-value storage of information also indicates the necessity of careful control of the electric charge injection by taking into account the degree of deterioration of the tunneling insulation film. Otherwise, erroneous operation will be caused.
A ferroelectric random access memory device is a semiconductor memory device having a ferroelectric capacitor that uses a ferroelectric film as the capacitor insulation film and stores information in the ferroelectric film in the form of spontaneous polarization of the ferroelectric film. In a ferroelectric random access memory device, writing or erasing of information is achieved by inverting the direction of the foregoing spontaneous polarization of the ferroelectric capacitor insulation film. Such an inversion of the polarization is achieved only by way of application of electric voltage and injection of electric current is not necessary. Thus, a ferroelectric random access memory device provides the advantageous features of very fast speed of writing operation and small power consumption. Further, in view of the fact that the polarization of the ferroelectric capacitor is limited either to a positive direction or a negative direction, there arises no problem of excessive erasing as in the case of a flash memory device.
FIG. 1 shows the construction of a conventional memory cell of a conventional ferroelectric random access memory device.
Referring to FIG. 1, the memory cell has a so called 2T/2C construction that uses two transfer gate transistors T1 and T2 and two ferroelectric capacitors C1 and C2 for storing one-bit information. In the construction of FIG. 1, the memory cell achieves a complementary operation in which information xe2x80x9c1xe2x80x9d is stored in one of the capacitors and information xe2x80x9c0xe2x80x9d is stored in the other capacitor.
In more detail, the transfer gate transistors T1 and T2 are turned on by selecting a word line WL, and the information xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d is written into the capacitor C1 from a bit line BIT connected to the transistor T1. Simultaneously, the complementary information xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d is written into the capacitor C2 from a complementary bit line /BIT. Thereby, the ferroelectric capacitor insulation films of the capacitors C1 and C2 store the written information in the form of spontaneous polarization.
In the reading mode operation of the memory cell, the word line WL is selected again and the transistors T1 and T2 are turned on. Further, the voltage difference appearing across the bit lines BIT and /BIT as a result of the respective polarizations of the ferroelectric capacitors C1 and C2 is detected by a sense amplifier S/A.
Generally, a ferroelectric material having a perovskite crystal structure such as PZT having a composition (Pb,Zr)TiO3 or PLZT having a composition (Pb, Zr) (Ti,La)O3 is used for the ferroelectric capacitor insulation film in the capacitors C1 and C2. Alternatively, a Bi compound having a layered structure such as SrBi2Ta2O9 designated as SBT or a compound represented as SrBi2(Ta,Nb)2O9 designated as SBTN may be used for the ferroelectric capacitor insulation film.
When forming such a ferroelectric capacitor, it is generally practiced to deposit the ferroelectric film by a sol-gel process or sputtering process in the form of amorphous phase. The amorphous film thus formed is then subjected to a crystallization process by applying a high-temperature annealing process for a very short time. Without crystallization, the film does not provide the desired ferroelectric polarization.
In such a crystallizing process, there is a tendency that the PZT or PLZT film undergoes oxygen defect formation. Thus, in order to avoid the oxygen defect formation and to avoid deterioration of the ferroelectric property, it is practiced to carry out the crystallization process in an oxidizing atmosphere. Thereby, in order to avoid the unwanted problem of oxidation of the lower electrode of the ferroelectric capacitor, there is a proposal to carry out the crystallization process first in an inert atmosphere and then in an oxidizing atmosphere.
After the foregoing crystallization and oxygen compensation process, the upper electrode of the ferroelectric capacitor is formed on the ferroelectric film thus processed. Conventionally, such a formation of the upper electrode has been achieved by sputtering a refractory metal film such as a Pt film or an Ir film. As the sputtering process of the Pt film or Ir film is conducted in a non-oxidizing atmosphere, there has been a problem that the oxygen defects are formed again in the ferroelectric film with the deposition of the upper electrode.
In relation to the problem of oxygen-defect formation at the time of deposition of the upper electrode, there has been a proposal to use a conductive oxide film such as an IrO2 film for the upper electrode of the ferroelectric capacitor. By using such a conductive oxide film for the upper electrode, the problem of oxygen-defect formation in the ferroelectric capacitor insulation film at the time of deposition of the upper electrode is successfully avoided by conducting the deposition of the upper electrode in an oxidizing atmosphere, and the problem of increase of the resistance of the upper electrode caused by the oxidation is also avoided as a result of use of oxide for the electrode.
Meanwhile, it has been known that the process of forming an IrO2 electrode film tends to induce the problem of abnormal growth of the IrO2 crystals leading to the formation of giant IrO2 crystals. Such giant IrO2 crystals act as defect in the IrO2 electrode and cause a decrease of yield of production of the semiconductor device. Further, electric properties of the ferroelectric capacitor are deteriorated by the existence of such giant IrO2 crystals.
Accordingly, it is a general object of the present invention to provide a novel and useful ferroelectric capacitor and a semiconductor device having such a ferroelectric capacitor wherein the foregoing problems are eliminated.
Another and more specific object of the present invention is to provide a ferroelectric capacitor having an upper electrode formed of a conductive oxide wherein the amount of switching electric charges of a ferroelectric film constituting the capacitor insulation film of the ferroelectric capacitor is increased.
Another object of the present invention is to provide a semiconductor device having a ferroelectric capacitor in which the amount of switching electric charges is increased.
Another object of the present invention is to provide a fabrication process of a semiconductor device having a ferroelectric capacitor wherein abnormal growth of IrO2 crystals in an IrO2 upper electrode is successfully suppressed.
Another object of the present invention is to provide a method of fabricating a ferroelectric capacitor, comprising the steps of:
forming a lower electrode;
forming a ferroelectric film on said lower electrode; and
forming an upper electrode on said ferroelectric film,
said step of forming said upper electrode comprising a first reactive sputtering process of a conductive oxide film and a second reactive sputtering process of said conductive oxide film conducted after said first reactive sputtering process,
said first and second reactive sputtering process being conducted by using a target of a metal element constituting said conductive oxide film,
said first reactive sputtering process being conducted under a first, oxidizing condition such that oxidation of said metal element takes place, said second reactive sputtering process being conducted under a second, less oxidizing condition.
Another object of the present invention is to provide a ferroelectric capacitor, comprising:
a substrate,
an active device formed on said substrate,
an interlayer insulation film provided on said substrate so as to cover said active device; and
a ferroelectric capacitor provided on said interlayer insulation film in electrical connection with said active device,
said ferroelectric capacitor comprising: a lower electrode formed on said interlayer insulation film, a ferroelectric film containing Pb formed on said lower electrode; and an upper electrode formed on said ferroelectric film, said upper electrode comprising a conductive oxide film deposited on said ferroelectric film, said conductive oxide film containing Pb with a generally uniform concentration in a thickness direction thereof, said conductive oxide film comprising a lower part and an upper part, said upper part containing more proportion of said metal in a metallic state as compared with said lower part.
According to the present invention, the problem of oxygen defect formation in the ferroelectric film at the time of formation of the upper electrode is successfully eliminated by forming the upper electrode by conducting a sputtering process of a conductive oxide film under an oxidizing condition. By reducing the thickness of the conductive oxide film thus formed, the abnormal growth of giant crystals in the conductive oxide film is suppressed, and the yield of production of the semiconductor device is improved substantially. Further, it becomes possible to deposit the conductive oxide film with an increased deposition rate without deteriorating the electric properties of the ferroelectric film, by depositing the conductive oxide film first with a strong oxidizing condition, followed by a less strong oxidizing condition. The conductive oxide film thus formed by such a two-step process that uses the strong oxidizing condition and less strong oxidizing condition contains Pb that constitutes the ferroelectric film, with a generally uniform concentration level in the thickness direction of the conductive oxide film.
Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings.